Jena E Johnson


Caltech Geology and Planetary Sciences

MC 170-25 1200 E. California Blvd, Pasadena, CA 91125

Research

I'm a fourth-year graduate student in the Fischer lab. I investigate the ancient and modern manganese cycle using a variety of geochemical techniques including scanning electron microscopy, secondary ion mass spectrometry, x-ray diffraction, and x-ray absorption spectroscopy with Samuel Webb.

Manganese is intimately tied to oxygen, the incredibly important molecule that we all breathe. This is because manganese enters the liquid Earth from volcanoes as reduced and soluble Mn (Mn2+). This reduced manganese (Mn2+) can only be oxidized in two ways: first, oxidized by O2 itself (because of the high redox potential of manganese, only oxygen and oxygen-related species like superoxide and oxygen radicals can oxidize manganese), or by the same photosystem that can oxidize water (Photosystem II) in cyanobacteria, plants and algae - Photosystem II oxidizes Mn2+ to Mn3+ before it oxidizes H2O to O2. What’s really amazing about the manganese cycle is that (like iron) while reduced manganese is soluble, oxidized manganese is insoluble and forms precipitates that are preserved in the rock record. See below:

So we can look at the geologic record of manganese and learn about both the history of O2 and the history of photosynthesis. Oxygen was at extremely low levels before the origin of water-oxidizing photosynthesis (which produces oxygen as a waste product). Part of my thesis investigates the manganese deposits associated with and subsequent to the rise of oxygen. Another part of my thesis focuses on manganese deposits before oxygen was abundant, since the oxidation of manganese by Photosystem II has led to the hypothesis that manganese-oxidizing photosynthesis was an evolutionary step prior to water-oxidizing photosynthesis. Looking carefully at significant manganese deposits from 2.415 billion years ago in South Africa (>50km and tens of weight percent over 200m of core), we determined that the manganese was original to the rocks. We then looked at two proxies for environmental oxygen - redox-sensitive detrital grains (like pyrite and uraninite) and multiple sulfur isotopes, both of which are extremely sensitive to the presence of oxygen. Both showed that oxygen had not yet risen at 2.415 billion years ago - suggesting the manganese deposits are evidence for manganese-oxidizing photosynthesis.

Both these earliest manganese deposits and the post-oxygen manganese deposits that we’ve looked at also tell interesting redox story. While modern manganese deposits are uniformly Mn(IV), all ancient manganese deposits are comprised of MnII and MnIII minerals. Our observations of these significantly more reduced minerals has led to to a project in collaboration with Ken Nealson and Ben Kocar. We’ve been using the common metal-reducing microbe Shewanella oneidensis to understand more about how microbial manganese(IV) reduction works and what secondary minerals are formed. We've designed a flow-through system at the x-ray beamline to monitor how the manganese mineralogy is changing during the reduction process.